Refill Friction Stir Spot Welding of a Graphene-Reinforced AA 6061 Aluminum Alloy

Solid-state refill friction stir spot welding (RFSSW) has been used to join a graphene-reinforced AA 6061. The effect of tool rotating speed on the microstructure and mechanical properties of the welded joints was studied, and the results show that RFSSW is suitable to join graphene-reinforced aluminum alloy and that sound joints can be obtained using a wide parameter range. Changing the tool rotating speed from 1600 to 2200 rpm hardly changed the joint cross-section morphology. Like a traditional joint, the graphene-reinforced AA 6061 RFSSW joint can be divided into a heat-affected zone, a thermo-mechanically affected zone, and stir zone, and they all have different grain sizes, orientations, and recrystallization ratios. Compared with the pin-affected zone, the sleeve-affected zone has larger grain sizes, a higher low angle boundary ratio, and a higher recrystallization ratio. The graphene reacts with Mg, forming 974619Mg2C3 phase. A higher rotating speed contributes to the joint hardness. The joint lap shear failure load first increases and then decreases with increasing the rotating speed. The maximum failure load of 5178 N was obtained when using 1800 rpm. All the joints present a tensile-shear failure mode.